Such a training device is known from EP 0 853 961 B1. On such a training device, a computing unit supplies a frequency converter with setpoint values for the amperage and the frequency of the current of a three phase AC motor provided for generating torque. The computing unit is supplied with the output signal of a position sensor measuring the position of a motor-driven crank which acts as a training element. By means of stored tables containing all the relevant machine-specific parameters, the computing unit calculates from the position value the values of the amperage and frequency of the motor current required for a desired course of the torque over position.
This known training device works in a quite satisfactory manner, but can be further improved with regard to certain functional requirements. Especially the use of such training devices in medical rehabilitation centres thus requires both high accuracy in maintaining a desired torque and limit stops which can precisely be adjusted for the range of movement of the training element. Such limit stops are particularly significant, for example, when the maximum deflection angle of a joint of the body after surgery is to be restored to its normal value by means of workout exercises in defined steps.
A training device having a three phase AC motor for generating torque is known from FR 2 709 067 A1, where both the speed of rotation of the motor is measured by a frequency-analogue rate sensor and the torque output is measured by a force sensor. The speed of rotation measured is used for controlling the frequency and the torque measured is used for controlling the motor current. The concept of this training device thus comprises two sensors and two interconnected control loops and its implementation therefore involves relatively high design complexity. Measuring the force by a sensor further involves potential problems such as the effects of temperature, drift as well as malfunctions resulting from vibration or impacts.